1. Field of Invention
The present invention relates to a light emitting device driver circuit and a method for driving a light emitting device; particularly, it relates to a light emitting device driver circuit and a method for driving a light emitting device, which require less number of circuit devices and the circuit devices can be made of devices of lower voltage sustaining specification.
2. Description of Related Art
Referring to FIG. 1, conventionally, to provide power to a light emitting device circuit from an AC power supply, it requires an AC-DC power regulator 10 to convert an AC voltage to a DC voltage, and a light emitting device driver circuit 20 to provide electrical power to the light emitting device circuit 50 and control current through the light emitting devices. The AC-DC power regulator 10 comprises a transformer 13, a primary side circuit 11, and a secondary side circuit 12. The secondary side circuit 12 detects the output voltage DC OUT, and provides a feedback signal to a pulse width modulation (PWM) controller PWM in the primary side circuit 11 by means of opto-coupling, to control the operation of a power switch P in the primary side circuit 11.
The aforementioned prior art has the following drawbacks. Because it requires the AC-DC power regulator 10 to generate a regulated voltage and the light emitting device driver circuit 20 to control current through the light emitting device circuit 50 according to the regulated voltage, the prior art circuitry needs at least three integrated circuit (IC) chips: the primary side circuit 11, the secondary side circuit 12, and the light emitting device driver circuit 20; this is not cost-effective. Besides, when the light emitting device circuit 50 requires high DC output voltage, the devices in the secondary side circuit 12, in the light emitting device driver circuit 20, and in the light emitting device circuit 50 which may possibly contact the high DC output voltage should be made of devices capable of sustaining such high voltage. Therefore, both the number of the circuit devices and the high voltage sustaining specification result in higher cost.
In view of the above, it is desired to provide a light emitting device driver circuit and a method for driving a light emitting device without these drawbacks.